Digital cameras, either still or video, utilize image sensors to capture digital image information. As with conventional film based cameras, digital cameras include imaging optics to focus light onto the image capture medium.
In order to reduce assembly time and cost for high volume digital cameras, it is important that the number of circuit boards required to mount all of the components, including the image sensor and imaging optics, be kept to a minimum and that the components have as small a footprint as possible. In addition, it is important that the electronic components can be attached to the circuit boards utilizing standard automated mounting techniques, such as widely utilized surface mounting techniques.
It is also important in digital camera applications that the imaging optics be accurately aligned with the image sensor so that the image bearing light is focused onto the image sensor. One technique for accurately aligning the imaging optics with the image sensor involves powering up the image sensor and adjusting the location of the imaging optics in response to a detected image. Powering up the image sensor to align the imaging optics is time consuming and requires specialized alignment equipment. Other techniques for accurately aligning the image sensor with the imaging optics do not involve powering up the image sensor.
Various digital image capture systems having an image sensor and imaging optics that can be aligned without powering up the imaging sensor have been adapted for high volume camera production. FIG. 1 depicts a first prior art digital image capture system which has an image sensor package 102 that is surface mounted to a circuit board 104, with the circuit board being attached to the outer case 106 of a digital camera. The imaging optics 108 are also connected to the outer case of the digital camera in a manner which allows image bearing light to contact an image sensor. The digital image capture system of FIG. 1 includes the outer case, the printed circuit board, the image sensor package, an image sensor chip 110, package leads 112, an image sensor cover glass 114, a lens holder 118, lens elements 120, a lens aperture 122, and an infra-red filter 124. Although the arrangement of image capture elements works well, accurate alignment of the lens elements with the image sensor is made difficult because of the distance between the lens elements and the image sensor and because the outer case typically has slight flexibility that can change the alignment. In addition, the outer case must be light-tight to ensure that unwanted light does not reach the image sensor.
FIG. 2 depicts a second prior art digital image capture system in which an image sensor package 202 is attached to a circuit board 204 and imaging optics 208 are attached directly to the same circuit board around the outer edges of the image sensor package. The digital image capture system of FIG. 2 includes the circuit board, the image sensor package, an image sensor chip 210, connection leads 212, an image sensor cover glass 214, a light shield 226, an infra-red filter 224, a filter retaining ring 228, a lens attachment structure 230, lens attachment screws 232, a lens holder 218, lens elements 220, and a lens aperture 222. Although the arrangement of image capture elements works well, the circuit board must be designed and built with precisely aligned attachment holes for attaching the imaging optics. Even with precisely aligned attachment holes for the imaging optics, the image sensor package must be mounted on the circuit board with precise accuracy in order to ensure alignment between the imaging optics and the image sensor. In addition, the attachment structure of the imaging optics extends beyond the outer edges of the image sensor package, thereby taking up valuable circuit board real estate.
FIG. 3 depicts a third prior art digital image capture system in which an alignment plate 334 is utilized to ensure alignment between an image sensor package 302 and imaging optics 308. The digital image capture system includes a circuit board 304, the alignment plate, the image sensor package, an image sensor chip 310, connection leads 312, an image sensor cover glass 314, a light shield 326, an infra-red filter 324, a filter retaining ring 328, a lens attachment structure 330, connection nuts 340 and bolts 342, a lens holder 318, lens elements 320, and a lens aperture 322. The alignment plate is located between the circuit board and the image sensor package and the imaging optics. The alignment plate includes through holes that enable the connection leads of the image sensor package to connect to the circuit board. The alignment plate also includes through holes for aligning and attaching the lens attachment structure to the circuit board. Because the image sensor package and the lens attachment structure are aligned with the alignment plate, the image sensor chip and the imaging optics are indirectly aligned with each other. Although the arrangement of image capture elements works well to improve the alignment problem described with reference to the image capture system of FIG. 2, the alignment plate is not compatible with surface mounting techniques that are utilized to attach image sensing packages in high volume assembly processes. In addition, the alignment plate adds extra cost to the image capture system.
In all of the above-described digital image capture systems, the image sensor is sealed within a package cavity by a cover glass. The cover glass protects the image sensor from damage while the image sensor package is soldered to the circuit board. Other techniques for attaching and aligning imaging optics with an image sensor involve attaching the imaging optics directly to a surface of the image sensor. While this provides some advantages, the imaging optics must be attached to the image sensor in a clean-room environment because the image sensor is very sensitive to environmental contaminants. Because the imaging optics must be attached to the image sensor in a clean-room environment, the imaging optics are typically attached to the image sensor before the image sensor is soldered to a circuit board. In order to minimize the exposure of the image sensor and the imaging optics to the high temperatures required for surface mounting techniques, some image sensor packages and imaging optics are through hole mounted to the circuit boards and soldered from the back, often by labor intensive hand soldering.
The description of prior art image capture systems is not meant to be exhaustive although it does highlight some of the problems involved with assembling image capture systems, such as high volume digital cameras, that utilize image sensor chips and imaging optics. In view of the problems involved with assembling image capture systems, what is needed is an economic technique for assembling an image capture system that ensures precise alignment between the image sensor and the imaging optics without having to power up the image sensor, without exposing the image sensor to contaminants, and without exposing the imaging optics to the high temperatures of the surface mounting process.